introduce DynSend and DynSync auto trait

Cargo.lock

@@ -3299,6 +3299,7 @@ dependencies = [
  "rustc-hash",
  "rustc-rayon",
  "rustc-rayon-core",
+ "rustc_arena",
  "rustc_graphviz",
  "rustc_index",
  "rustc_macros",

compiler/rustc_ast/src/tokenstream.rs

@@ -48,14 +48,15 @@ pub enum TokenTree {
     Delimited(DelimSpan, Delimiter, TokenStream),
 }
 
-// Ensure all fields of `TokenTree` is `Send` and `Sync`.
+// Ensure all fields of `TokenTree` is `DynSend` and `DynSync`.
 #[cfg(parallel_compiler)]
 fn _dummy()
 where
-    Token: Send + Sync,
+    Token: sync::DynSend + sync::DynSync,
-    DelimSpan: Send + Sync,
+    Spacing: sync::DynSend + sync::DynSync,
-    Delimiter: Send + Sync,
+    DelimSpan: sync::DynSend + sync::DynSync,
-    TokenStream: Send + Sync,
+    Delimiter: sync::DynSend + sync::DynSync,
+    TokenStream: sync::DynSend + sync::DynSync,
 {
 }
 
@@ -118,7 +119,7 @@ where
     }
 }
 
-pub trait ToAttrTokenStream: sync::Send + sync::Sync {
+pub trait ToAttrTokenStream: sync::DynSend + sync::DynSync {
     fn to_attr_token_stream(&self) -> AttrTokenStream;
 }
 

compiler/rustc_codegen_ssa/src/base.rs

@@ -17,10 +17,7 @@ use rustc_ast::expand::allocator::AllocatorKind;
 use rustc_attr as attr;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::profiling::{get_resident_set_size, print_time_passes_entry};
-
+use rustc_data_structures::sync::par_map;
-use rustc_data_structures::sync::par_iter;
-#[cfg(parallel_compiler)]
-use rustc_data_structures::sync::ParallelIterator;
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 use rustc_hir::lang_items::LangItem;
@@ -689,7 +686,7 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
     // This likely is a temporary measure. Once we don't have to support the
     // non-parallel compiler anymore, we can compile CGUs end-to-end in
     // parallel and get rid of the complicated scheduling logic.
-    let mut pre_compiled_cgus = if cfg!(parallel_compiler) {
+    let mut pre_compiled_cgus = if tcx.sess.threads() > 1 {
         tcx.sess.time("compile_first_CGU_batch", || {
             // Try to find one CGU to compile per thread.
             let cgus: Vec<_> = cgu_reuse
@@ -702,12 +699,10 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
             // Compile the found CGUs in parallel.
             let start_time = Instant::now();
 
-            let pre_compiled_cgus = par_iter(cgus)
+            let pre_compiled_cgus = par_map(cgus, |(i, _)| {
-                .map(|(i, _)| {
+                let module = backend.compile_codegen_unit(tcx, codegen_units[i].name());
-                    let module = backend.compile_codegen_unit(tcx, codegen_units[i].name());
+                (i, module)
-                    (i, module)
+            });
-                })
-                .collect();
 
             total_codegen_time += start_time.elapsed();
 

compiler/rustc_codegen_ssa/src/traits/backend.rs

@@ -22,6 +22,7 @@ use rustc_target::spec::Target;
 
 pub use rustc_data_structures::sync::MetadataRef;
 
+use rustc_data_structures::sync::{DynSend, DynSync};
 use std::any::Any;
 
 pub trait BackendTypes {
@@ -117,7 +118,9 @@ pub trait CodegenBackend {
     ) -> Result<(), ErrorGuaranteed>;
 }
 
-pub trait ExtraBackendMethods: CodegenBackend + WriteBackendMethods + Sized + Send + Sync {
+pub trait ExtraBackendMethods:
+    CodegenBackend + WriteBackendMethods + Sized + Send + Sync + DynSend + DynSync
+{
     fn codegen_allocator<'tcx>(
         &self,
         tcx: TyCtxt<'tcx>,

compiler/rustc_data_structures/Cargo.toml

@@ -16,6 +16,7 @@ libc = "0.2"
 measureme = "10.0.0"
 rustc-rayon-core = { version = "0.5.0", optional = true }
 rustc-rayon = { version = "0.5.0", optional = true }
+rustc_arena = { path = "../rustc_arena" }
 rustc_graphviz = { path = "../rustc_graphviz" }
 rustc-hash = "1.1.0"
 rustc_index = { path = "../rustc_index", package = "rustc_index" }

compiler/rustc_data_structures/src/lib.rs

@@ -26,9 +26,11 @@
 #![feature(test)]
 #![feature(thread_id_value)]
 #![feature(vec_into_raw_parts)]
+#![feature(allocator_api)]
 #![feature(get_mut_unchecked)]
 #![feature(lint_reasons)]
-#![feature(unwrap_infallible)]
+#![feature(unwrap_infallible)]#![feature(const_mut_refs)]
+#![feature(const_trait_impl)]
 #![feature(strict_provenance)]
 #![feature(ptr_alignment_type)]
 #![feature(macro_metavar_expr)]
@@ -77,6 +79,7 @@ pub mod sorted_map;
 pub mod stable_hasher;
 mod atomic_ref;
 pub mod fingerprint;
+pub mod marker;
 pub mod profiling;
 pub mod sharded;
 pub mod stack;

compiler/rustc_data_structures/src/marker.rs

@@ -0,0 +1,268 @@
+cfg_if!(
+    if #[cfg(not(parallel_compiler))] {
+        pub auto trait DynSend {}
+        pub auto trait DynSync {}
+
+        impl<T> DynSend for T {}
+        impl<T> DynSync for T {}
+    } else {
+        #[rustc_on_unimplemented(
+            message = "`{Self}` doesn't implement `DynSend`. \
+            Add it to `rustc_data_structures::marker` or use `IntoDyn` if it's already `Send`",
+            label = "`{Self}` doesn't implement `DynSend`. \
+            Add it to `rustc_data_structures::marker` or use `IntoDyn` if it's already `Send`"
+        )]
+        // Ensure data structures is `Send` if `sync::active()` is true.
+        // `sync::active()` should be checked before using these data structures.
+        // Note: Ensure that the data structure **will not break**
+        // thread safety after being created.
+        //
+        // `sync::active()` should be checked when downcasting these data structures
+        // to `Send` via `FromDyn`.
+        pub unsafe auto trait DynSend {}
+
+        #[rustc_on_unimplemented(
+            message = "`{Self}` doesn't implement `DynSync`. \
+            Add it to `rustc_data_structures::marker` or use `IntoDyn` if it's already `Sync`",
+            label = "`{Self}` doesn't implement `DynSync`. \
+            Add it to `rustc_data_structures::marker` or use `IntoDyn` if it's already `Sync`"
+        )]
+        // Ensure data structures is `Sync` if `sync::active()` is true.
+        // Note: Ensure that the data structure **will not break**
+        // thread safety after being checked.
+        //
+        // `sync::active()` should be checked when downcasting these data structures
+        // to `Send` via `FromDyn`.
+        pub unsafe auto trait DynSync {}
+
+        // Same with `Sync` and `Send`.
+        unsafe impl<T: DynSync + ?Sized> DynSend for &T {}
+
+        macro_rules! impls_dyn_send_neg {
+            ($([$t1: ty $(where $($generics1: tt)*)?])*) => {
+                $(impl$(<$($generics1)*>)? !DynSend for $t1 {})*
+            };
+        }
+
+        // Consistent with `std`
+        impls_dyn_send_neg!(
+            [std::env::Args]
+            [std::env::ArgsOs]
+            [*const T where T: ?Sized]
+            [*mut T where T: ?Sized]
+            [std::ptr::NonNull<T> where T: ?Sized]
+            [std::rc::Rc<T> where T: ?Sized]
+            [std::rc::Weak<T> where T: ?Sized]
+            [std::sync::MutexGuard<'_, T> where T: ?Sized]
+            [std::sync::RwLockReadGuard<'_, T> where T: ?Sized]
+            [std::sync::RwLockWriteGuard<'_, T> where T: ?Sized]
+            [std::io::StdoutLock<'_>]
+            [std::io::StderrLock<'_>]
+        );
+        cfg_if!(
+            // Consistent with `std`
+            // `os_imp::Env` is `!Send` in these platforms
+            if #[cfg(any(unix, target_os = "hermit", target_os = "wasi", target_os = "solid_asp3"))] {
+                impl !DynSend for std::env::VarsOs {}
+            }
+        );
+
+        macro_rules! already_send {
+            ($([$ty: ty])*) => {
+                $(unsafe impl DynSend for $ty where $ty: Send {})*
+            };
+        }
+
+        // These structures are already `Send`.
+        already_send!(
+            [std::backtrace::Backtrace]
+            [std::io::Stdout]
+            [std::io::Stderr]
+            [std::io::Error]
+            [std::fs::File]
+            [rustc_arena::DroplessArena]
+            [crate::memmap::Mmap]
+            [crate::profiling::SelfProfiler]
+        );
+
+        macro_rules! impl_dyn_send {
+            ($($($attr: meta)* [$ty: ty where $($generics2: tt)*])*) => {
+                $(unsafe impl<$($generics2)*> DynSend for $ty {})*
+            };
+        }
+
+        impl_dyn_send!(
+            [std::sync::atomic::AtomicPtr<T> where T]
+            [std::sync::Mutex<T> where T: ?Sized+ DynSend]
+            [std::sync::mpsc::Sender<T> where T: DynSend]
+            [std::sync::Arc<T> where T: ?Sized + DynSync + DynSend]
+            [std::sync::LazyLock<T, F> where T: DynSend, F: DynSend]
+            [std::collections::HashSet<K, S> where K: DynSend, S: DynSend]
+            [std::collections::HashMap<K, V, S> where K: DynSend, V: DynSend, S: DynSend]
+            [std::collections::BTreeMap<K, V, A> where K: DynSend, V: DynSend, A: std::alloc::Allocator + Clone + DynSend]
+            [Vec<T, A> where T: DynSend, A: std::alloc::Allocator + DynSend]
+            [Box<T, A> where T: ?Sized + DynSend, A: std::alloc::Allocator + DynSend]
+            [crate::sync::Lock<T> where T: DynSend]
+            [crate::sync::RwLock<T> where T: DynSend]
+            [rustc_arena::TypedArena<T> where T: DynSend]
+            [indexmap::IndexSet<V, S> where V: DynSend, S: DynSend]
+            [indexmap::IndexMap<K, V, S> where K: DynSend, V: DynSend, S: DynSend]
+            [thin_vec::ThinVec<T> where T: DynSend]
+            [smallvec::SmallVec<A> where A: smallvec::Array + DynSend]
+
+            // We use `Send` here to omit some extra code, since they are only
+            // used in `Send` situations now.
+            [crate::owning_ref::OwningRef<O, T> where O: Send, T: ?Sized + Send]
+            [crate::owning_ref::OwningRefMut<O, T> where O: Send, T: ?Sized + Send]
+        );
+
+        macro_rules! impls_dyn_sync_neg {
+            ($([$t1: ty $(where $($generics1: tt)*)?])*) => {
+                $(impl$(<$($generics1)*>)? !DynSync for $t1 {})*
+            };
+        }
+
+        // Consistent with `std`
+        impls_dyn_sync_neg!(
+            [std::env::Args]
+            [std::env::ArgsOs]
+            [*const T where T: ?Sized]
+            [*mut T where T: ?Sized]
+            [std::cell::Cell<T> where T: ?Sized]
+            [std::cell::RefCell<T> where T: ?Sized]
+            [std::cell::UnsafeCell<T> where T: ?Sized]
+            [std::ptr::NonNull<T> where T: ?Sized]
+            [std::rc::Rc<T> where T: ?Sized]
+            [std::rc::Weak<T> where T: ?Sized]
+            [std::cell::OnceCell<T> where T]
+            [std::sync::mpsc::Receiver<T> where T]
+            [std::sync::mpsc::Sender<T> where T]
+        );
+        cfg_if!(
+            // Consistent with `std`
+            // `os_imp::Env` is `!Sync` in these platforms
+            if #[cfg(any(unix, target_os = "hermit", target_os = "wasi", target_os = "solid_asp3"))] {
+                impl !DynSync for std::env::VarsOs {}
+            }
+        );
+
+        macro_rules! already_sync {
+            ($([$ty: ty])*) => {
+                $(unsafe impl DynSync for $ty where $ty: Sync {})*
+            };
+        }
+
+        // These structures are already `Sync`.
+        already_sync!(
+            [std::sync::atomic::AtomicBool]
+            [std::sync::atomic::AtomicUsize]
+            [std::sync::atomic::AtomicU8]
+            [std::sync::atomic::AtomicU32]
+            [std::sync::atomic::AtomicU64]
+            [std::backtrace::Backtrace]
+            [std::io::Error]
+            [std::fs::File]
+            [jobserver_crate::Client]
+            [crate::memmap::Mmap]
+            [crate::profiling::SelfProfiler]
+        );
+
+        macro_rules! impl_dyn_sync {
+            ($($($attr: meta)* [$ty: ty where $($generics2: tt)*])*) => {
+                $(unsafe impl<$($generics2)*> DynSync for $ty {})*
+            };
+        }
+
+        impl_dyn_sync!(
+            [std::sync::atomic::AtomicPtr<T> where T]
+            [std::sync::OnceLock<T> where T: DynSend + DynSync]
+            [std::sync::Mutex<T> where T: ?Sized + DynSend]
+            [std::sync::Arc<T> where T: ?Sized + DynSync + DynSend]
+            [std::sync::LazyLock<T, F> where T: DynSend + DynSync, F: DynSend]
+            [std::collections::HashSet<K, S> where K: DynSync, S: DynSync]
+            [std::collections::HashMap<K, V, S> where K: DynSync, V: DynSync, S: DynSync]
+            [std::collections::BTreeMap<K, V, A> where K: DynSync, V: DynSync, A: std::alloc::Allocator + Clone + DynSync]
+            [Vec<T, A> where T: DynSync, A: std::alloc::Allocator + DynSync]
+            [Box<T, A> where T: ?Sized + DynSync, A: std::alloc::Allocator + DynSync]
+            [crate::sync::Lock<T> where T: DynSend]
+            [crate::sync::RwLock<T> where T: DynSend + DynSync]
+            [crate::sync::OneThread<T> where T]
+            [crate::sync::WorkerLocal<T> where T: DynSend]
+            [crate::intern::Interned<'a, T> where 'a, T: DynSync]
+            [parking_lot::lock_api::Mutex<R, T> where R: DynSync, T: ?Sized + DynSend]
+            [parking_lot::lock_api::RwLock<R, T> where R: DynSync, T: ?Sized + DynSend + DynSync]
+            [indexmap::IndexSet<V, S> where V: DynSync, S: DynSync]
+            [indexmap::IndexMap<K, V, S> where K: DynSync, V: DynSync, S: DynSync]
+            [smallvec::SmallVec<A> where A: smallvec::Array + DynSync]
+            [thin_vec::ThinVec<T> where T: DynSync]
+
+            // We use `Sync` here to omit some extra code, since they are only
+            // used in `Sync` situations now.
+            [crate::owning_ref::OwningRef<O, T> where O: Sync, T: ?Sized + Sync]
+            [crate::owning_ref::OwningRefMut<O, T> where O: Sync, T: ?Sized + Sync]
+        );
+    }
+);
+
+pub fn assert_dyn_sync<T: ?Sized + DynSync>() {}
+pub fn assert_dyn_send<T: ?Sized + DynSend>() {}
+pub fn assert_dyn_send_val<T: ?Sized + DynSend>(_t: &T) {}
+pub fn assert_dyn_send_sync_val<T: ?Sized + DynSync + DynSend>(_t: &T) {}
+
+#[derive(Copy, Clone)]
+pub struct FromDyn<T>(T);
+
+impl<T> FromDyn<T> {
+    // Check `sync::active()` when creating this structure
+    // and downcasting to `Send`. So we can ensure it is
+    // thread-safe.
+    #[inline(always)]
+    pub fn from(val: T) -> Self {
+        #[cfg(parallel_compiler)]
+        assert!(crate::sync::active());
+        FromDyn(val)
+    }
+
+    #[inline(always)]
+    pub fn into_inner(self) -> T {
+        self.0
+    }
+}
+
+// `FromDyn` is `Send` if `T` is `DynSend`, since it check when created.
+#[cfg(parallel_compiler)]
+unsafe impl<T: DynSend> Send for FromDyn<T> {}
+
+// `FromDyn` is `Sync` if `T` is `DynSync`, since it check when created.
+#[cfg(parallel_compiler)]
+unsafe impl<T: DynSync> Sync for FromDyn<T> {}
+
+impl<T> const std::ops::Deref for FromDyn<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+#[derive(Copy, Clone)]
+pub struct IntoDyn<T: ?Sized>(pub T);
+
+#[cfg(parallel_compiler)]
+unsafe impl<T: ?Sized + Send> DynSend for IntoDyn<T> {}
+#[cfg(parallel_compiler)]
+unsafe impl<T: ?Sized + Sync> DynSync for IntoDyn<T> {}
+
+impl<T> const std::ops::Deref for IntoDyn<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &self.0
+    }
+}
+
+impl<T> const std::ops::DerefMut for IntoDyn<T> {
+    fn deref_mut(&mut self) -> &mut T {
+        &mut self.0
+    }
+}

compiler/rustc_data_structures/src/sync.rs

@@ -39,6 +39,7 @@
 //!
 //! [^2] `MTLockRef` is a typedef.
 
+pub use crate::marker::*;
 use crate::owned_slice::OwnedSlice;
 use std::collections::HashMap;
 use std::hash::{BuildHasher, Hash};
@@ -55,6 +56,37 @@ pub use vec::{AppendOnlyIndexVec, AppendOnlyVec};
 
 mod vec;
 
+mod mode {
+    use super::Ordering;
+    use std::sync::atomic::AtomicU8;
+
+    const UNINITIALIZED: u8 = 0;
+    const INACTIVE: u8 = 1;
+    const ACTIVE: u8 = 2;
+
+    static MODE: AtomicU8 = AtomicU8::new(UNINITIALIZED);
+
+    #[inline]
+    pub fn active() -> bool {
+        match MODE.load(Ordering::Relaxed) {
+            INACTIVE => false,
+            ACTIVE => true,
+            _ => panic!("uninitialized parallel mode!"),
+        }
+    }
+
+    // Only set by the `-Z threads` compile option
+    pub fn set(parallel: bool) {
+        let set: u8 = if parallel { ACTIVE } else { INACTIVE };
+        let previous =
+            MODE.compare_exchange(UNINITIALIZED, set, Ordering::Relaxed, Ordering::Relaxed);
+
+        // Check that the mode was either uninitialized or was already set to the requested mode.
+        assert!(previous.is_ok() || previous == Err(set));
+    }
+}
+
+pub use mode::{active, set};
 cfg_if! {
     if #[cfg(not(parallel_compiler))] {
         pub unsafe auto trait Send {}
@@ -149,7 +181,7 @@ cfg_if! {
 
         #[macro_export]
         macro_rules! parallel {
-            ($($blocks:tt),*) => {
+            ($($blocks:block),*) => {{
                 // We catch panics here ensuring that all the blocks execute.
                 // This makes behavior consistent with the parallel compiler.
                 let mut panic = None;
@@ -165,13 +197,7 @@ cfg_if! {
                 if let Some(panic) = panic {
                     ::std::panic::resume_unwind(panic);
                 }
-            }
+            }}
-        }
-
-        pub use Iterator as ParallelIterator;
-
-        pub fn par_iter<T: IntoIterator>(t: T) -> T::IntoIter {
-            t.into_iter()
         }
 
         pub fn par_for_each_in<T: IntoIterator>(t: T, mut for_each: impl FnMut(T::Item) + Sync + Send) {
@@ -190,6 +216,29 @@ cfg_if! {
             }
         }
 
+        pub fn par_map<T: IntoIterator, R, C: FromIterator<R>>(
+            t: T,
+            mut map: impl FnMut(<<T as IntoIterator>::IntoIter as Iterator>::Item) -> R,
+        ) -> C {
+            // We catch panics here ensuring that all the loop iterations execute.
+            let mut panic = None;
+            let r = t.into_iter().filter_map(|i| {
+                match catch_unwind(AssertUnwindSafe(|| map(i))) {
+                    Ok(r) => Some(r),
+                    Err(p) => {
+                        if panic.is_none() {
+                            panic = Some(p);
+                        }
+                        None
+                    }
+                }
+            }).collect();
+            if let Some(panic) = panic {
+                resume_unwind(panic);
+            }
+            r
+        }
+
         pub type MetadataRef = OwnedSlice;
 
         pub use std::rc::Rc as Lrc;
@@ -302,46 +351,159 @@ cfg_if! {
         use parking_lot::RwLock as InnerRwLock;
 
         use std::thread;
-        pub use rayon::{join, scope};
+
+        #[inline]
+        pub fn join<A, B, RA: DynSend, RB: DynSend>(oper_a: A, oper_b: B) -> (RA, RB)
+        where
+            A: FnOnce() -> RA + DynSend,
+            B: FnOnce() -> RB + DynSend,
+        {
+            if mode::active() {
+                let oper_a = FromDyn::from(oper_a);
+                let oper_b = FromDyn::from(oper_b);
+                let (a, b) = rayon::join(move || FromDyn::from(oper_a.into_inner()()), move || FromDyn::from(oper_b.into_inner()()));
+                (a.into_inner(), b.into_inner())
+            } else {
+                (oper_a(), oper_b())
+            }
+        }
+
+        pub fn scope<'scope, OP, R>(op: OP) -> R
+        where
+            OP: FnOnce(&rayon::Scope<'scope>) -> R + DynSend,
+            R: DynSend,
+        {
+            let op = FromDyn::from(op);
+            rayon::scope(|s| FromDyn::from(op.into_inner()(s))).into_inner()
+        }
 
         /// Runs a list of blocks in parallel. The first block is executed immediately on
         /// the current thread. Use that for the longest running block.
         #[macro_export]
         macro_rules! parallel {
-            (impl $fblock:tt [$($c:tt,)*] [$block:tt $(, $rest:tt)*]) => {
+            ($fblock:block [$($c:expr,)*] [$block:expr $(, $rest:expr)*]) => {
-                parallel!(impl $fblock [$block, $($c,)*] [$($rest),*])
+                parallel!($fblock [$block, $($c,)*] [$($rest),*])
             };
-            (impl $fblock:tt [$($blocks:tt,)*] []) => {
+            ($fblock:block [$($blocks:expr,)*] []) => {
-                ::rustc_data_structures::sync::scope(|s| {
+                {
+                    ::rustc_data_structures::sync::scope(|s| {
+                        $(let block = rustc_data_structures::sync::FromDyn::from(|| $blocks);
+                        s.spawn(move |_| block.into_inner()());)*
+                        (|| $fblock)();
+                    });
+                }
+            };
+            ($fblock:block, $($blocks:block),*) => {
+                if rustc_data_structures::sync::active() {
+                    // Reverse the order of the later blocks since Rayon executes them in reverse order
+                    // when using a single thread. This ensures the execution order matches that
+                    // of a single threaded rustc
+                    parallel!($fblock [] [$($blocks),*]);
+                } else {
+                    // We catch panics here ensuring that all the blocks execute.
+                    // This makes behavior consistent with the parallel compiler.
+                    let mut panic = None;
                     $(
-                        s.spawn(|_| $blocks);
+                        if let Err(p) = ::std::panic::catch_unwind(
+                            ::std::panic::AssertUnwindSafe(|| $blocks)
+                        ) {
+                            if panic.is_none() {
+                                panic = Some(p);
+                            }
+                        }
                     )*
-                    $fblock;
+                    if let Some(panic) = panic {
-                })
+                        ::std::panic::resume_unwind(panic);
-            };
+                    }
-            ($fblock:tt, $($blocks:tt),*) => {
+                }
-                // Reverse the order of the later blocks since Rayon executes them in reverse order
-                // when using a single thread. This ensures the execution order matches that
-                // of a single threaded rustc
-                parallel!(impl $fblock [] [$($blocks),*]);
             };
         }
 
-        pub use rayon::iter::ParallelIterator;
+        use rayon::iter::{FromParallelIterator, IntoParallelIterator, ParallelIterator};
-        use rayon::iter::IntoParallelIterator;
 
-        pub fn par_iter<T: IntoParallelIterator>(t: T) -> T::Iter {
+        pub fn par_for_each_in<I, T: IntoIterator<Item = I> + IntoParallelIterator<Item = I>>(
-            t.into_par_iter()
-        }
-
-        pub fn par_for_each_in<T: IntoParallelIterator>(
             t: T,
-            for_each: impl Fn(T::Item) + Sync + Send,
+            for_each: impl Fn(I) + DynSync + DynSend
         ) {
-            let ps: Vec<_> = t.into_par_iter().map(|i| catch_unwind(AssertUnwindSafe(|| for_each(i)))).collect();
+            if mode::active() {
-            ps.into_iter().for_each(|p| if let Err(panic) = p {
+                let for_each = FromDyn::from(for_each);
-                resume_unwind(panic)
+                let panic: Lock<Option<_>> = Lock::new(None);
-            });
+                t.into_par_iter().for_each(|i| if let Err(p) = catch_unwind(AssertUnwindSafe(|| for_each(i))) {
+                    let mut l = panic.lock();
+                    if l.is_none() {
+                        *l = Some(p)
+                    }
+                });
+
+                if let Some(panic) = panic.into_inner() {
+                    resume_unwind(panic);
+                }
+            } else {
+                // We catch panics here ensuring that all the loop iterations execute.
+                // This makes behavior consistent with the parallel compiler.
+                let mut panic = None;
+                t.into_iter().for_each(|i| {
+                    if let Err(p) = catch_unwind(AssertUnwindSafe(|| for_each(i))) {
+                        if panic.is_none() {
+                            panic = Some(p);
+                        }
+                    }
+                });
+                if let Some(panic) = panic {
+                    resume_unwind(panic);
+                }
+            }
+        }
+
+        pub fn par_map<
+            I,
+            T: IntoIterator<Item = I> + IntoParallelIterator<Item = I>,
+            R: std::marker::Send,
+            C: FromIterator<R> + FromParallelIterator<R>
+        >(
+            t: T,
+            map: impl Fn(I) -> R + DynSync + DynSend
+        ) -> C {
+            if mode::active() {
+                let panic: Lock<Option<_>> = Lock::new(None);
+                let map = FromDyn::from(map);
+                // We catch panics here ensuring that all the loop iterations execute.
+                let r = t.into_par_iter().filter_map(|i| {
+                    match catch_unwind(AssertUnwindSafe(|| map(i))) {
+                        Ok(r) => Some(r),
+                        Err(p) => {
+                            let mut l = panic.lock();
+                            if l.is_none() {
+                                *l = Some(p);
+                            }
+                            None
+                        },
+                    }
+                }).collect();
+
+                if let Some(panic) = panic.into_inner() {
+                    resume_unwind(panic);
+                }
+                r
+            } else {
+                // We catch panics here ensuring that all the loop iterations execute.
+                let mut panic = None;
+                let r = t.into_iter().filter_map(|i| {
+                    match catch_unwind(AssertUnwindSafe(|| map(i))) {
+                        Ok(r) => Some(r),
+                        Err(p) => {
+                            if panic.is_none() {
+                                panic = Some(p);
+                            }
+                            None
+                        }
+                    }
+                }).collect();
+                if let Some(panic) = panic {
+                    resume_unwind(panic);
+                }
+                r
+            }
         }
 
         pub type MetadataRef = OwnedSlice;
@@ -352,11 +514,6 @@ cfg_if! {
     }
 }
 
-pub fn assert_sync<T: ?Sized + Sync>() {}
-pub fn assert_send<T: ?Sized + Send>() {}
-pub fn assert_send_val<T: ?Sized + Send>(_t: &T) {}
-pub fn assert_send_sync_val<T: ?Sized + Sync + Send>(_t: &T) {}
-
 #[derive(Default)]
 #[cfg_attr(parallel_compiler, repr(align(64)))]
 pub struct CacheAligned<T>(pub T);

compiler/rustc_driver_impl/src/lib.rs

@@ -255,6 +255,9 @@ fn run_compiler(
 
     let sopts = config::build_session_options(&matches);
 
+    // Set parallel mode before thread pool creation as the session will already create locks.
+    interface::set_parallel_mode(&sopts.unstable_opts);
+
     if let Some(ref code) = matches.opt_str("explain") {
         handle_explain(diagnostics_registry(), code, sopts.error_format);
         return Ok(());

compiler/rustc_error_messages/src/lib.rs

@@ -11,7 +11,7 @@ extern crate tracing;
 use fluent_bundle::FluentResource;
 use fluent_syntax::parser::ParserError;
 use icu_provider_adapters::fallback::{LocaleFallbackProvider, LocaleFallbacker};
-use rustc_data_structures::sync::Lrc;
+use rustc_data_structures::sync::{IntoDyn, Lrc};
 use rustc_fluent_macro::fluent_messages;
 use rustc_macros::{Decodable, Encodable};
 use rustc_span::Span;
@@ -37,16 +37,17 @@ pub use unic_langid::{langid, LanguageIdentifier};
 
 fluent_messages! { "../messages.ftl" }
 
-pub type FluentBundle = fluent_bundle::bundle::FluentBundle<FluentResource, IntlLangMemoizer>;
+pub type FluentBundle =
-
+    IntoDyn<fluent_bundle::bundle::FluentBundle<FluentResource, IntlLangMemoizer>>;
-#[cfg(parallel_compiler)]
-fn new_bundle(locales: Vec<LanguageIdentifier>) -> FluentBundle {
-    FluentBundle::new_concurrent(locales)
-}
 
 #[cfg(not(parallel_compiler))]
 fn new_bundle(locales: Vec<LanguageIdentifier>) -> FluentBundle {
-    FluentBundle::new(locales)
+    IntoDyn(fluent_bundle::bundle::FluentBundle::new(locales))
+}
+
+#[cfg(parallel_compiler)]
+fn new_bundle(locales: Vec<LanguageIdentifier>) -> FluentBundle {
+    IntoDyn(fluent_bundle::bundle::FluentBundle::new_concurrent(locales))
 }
 
 #[derive(Debug)]

compiler/rustc_errors/src/lib.rs

@@ -32,7 +32,7 @@ use emitter::{is_case_difference, Emitter, EmitterWriter};
 use registry::Registry;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap, FxIndexSet};
 use rustc_data_structures::stable_hasher::{Hash128, StableHasher};
-use rustc_data_structures::sync::{self, Lock, Lrc};
+use rustc_data_structures::sync::{self, IntoDyn, Lock, Lrc};
 use rustc_data_structures::AtomicRef;
 pub use rustc_error_messages::{
     fallback_fluent_bundle, fluent_bundle, DelayDm, DiagnosticMessage, FluentBundle,
@@ -409,7 +409,7 @@ struct HandlerInner {
     err_count: usize,
     warn_count: usize,
     deduplicated_err_count: usize,
-    emitter: Box<dyn Emitter + sync::Send>,
+    emitter: IntoDyn<Box<dyn Emitter + sync::Send>>,
     delayed_span_bugs: Vec<DelayedDiagnostic>,
     delayed_good_path_bugs: Vec<DelayedDiagnostic>,
     /// This flag indicates that an expected diagnostic was emitted and suppressed.
@@ -605,7 +605,7 @@ impl Handler {
                 warn_count: 0,
                 deduplicated_err_count: 0,
                 deduplicated_warn_count: 0,
-                emitter,
+                emitter: IntoDyn(emitter),
                 delayed_span_bugs: Vec::new(),
                 delayed_good_path_bugs: Vec::new(),
                 suppressed_expected_diag: false,

compiler/rustc_errors/src/tests.rs

@@ -2,7 +2,7 @@ use crate::error::{TranslateError, TranslateErrorKind};
 use crate::fluent_bundle::*;
 use crate::translation::Translate;
 use crate::FluentBundle;
-use rustc_data_structures::sync::Lrc;
+use rustc_data_structures::sync::{IntoDyn, Lrc};
 use rustc_error_messages::fluent_bundle::resolver::errors::{ReferenceKind, ResolverError};
 use rustc_error_messages::langid;
 use rustc_error_messages::DiagnosticMessage;
@@ -27,10 +27,12 @@ fn make_dummy(ftl: &'static str) -> Dummy {
     let langid_en = langid!("en-US");
 
     #[cfg(parallel_compiler)]
-    let mut bundle = FluentBundle::new_concurrent(vec![langid_en]);
+    let mut bundle: FluentBundle =
+        IntoDyn(crate::fluent_bundle::bundle::FluentBundle::new_concurrent(vec![langid_en]));
 
     #[cfg(not(parallel_compiler))]
-    let mut bundle = FluentBundle::new(vec![langid_en]);
+    let mut bundle: FluentBundle =
+        IntoDyn(crate::fluent_bundle::bundle::FluentBundle::new(vec![langid_en]));
 
     bundle.add_resource(resource).expect("Failed to add FTL resources to the bundle.");
 

compiler/rustc_expand/src/base.rs

@@ -653,13 +653,13 @@ pub enum SyntaxExtensionKind {
     /// A token-based function-like macro.
     Bang(
         /// An expander with signature TokenStream -> TokenStream.
-        Box<dyn BangProcMacro + sync::Sync + sync::Send>,
+        Box<dyn BangProcMacro + sync::DynSync + sync::DynSend>,
     ),
 
     /// An AST-based function-like macro.
     LegacyBang(
         /// An expander with signature TokenStream -> AST.
-        Box<dyn TTMacroExpander + sync::Sync + sync::Send>,
+        Box<dyn TTMacroExpander + sync::DynSync + sync::DynSend>,
     ),
 
     /// A token-based attribute macro.
@@ -667,7 +667,7 @@ pub enum SyntaxExtensionKind {
         /// An expander with signature (TokenStream, TokenStream) -> TokenStream.
         /// The first TokenSteam is the attribute itself, the second is the annotated item.
         /// The produced TokenSteam replaces the input TokenSteam.
-        Box<dyn AttrProcMacro + sync::Sync + sync::Send>,
+        Box<dyn AttrProcMacro + sync::DynSync + sync::DynSend>,
     ),
 
     /// An AST-based attribute macro.
@@ -675,7 +675,7 @@ pub enum SyntaxExtensionKind {
         /// An expander with signature (AST, AST) -> AST.
         /// The first AST fragment is the attribute itself, the second is the annotated item.
         /// The produced AST fragment replaces the input AST fragment.
-        Box<dyn MultiItemModifier + sync::Sync + sync::Send>,
+        Box<dyn MultiItemModifier + sync::DynSync + sync::DynSend>,
     ),
 
     /// A trivial attribute "macro" that does nothing,
@@ -692,14 +692,14 @@ pub enum SyntaxExtensionKind {
         /// is handled identically to `LegacyDerive`. It should be migrated to
         /// a token-based representation like `Bang` and `Attr`, instead of
         /// using `MultiItemModifier`.
-        Box<dyn MultiItemModifier + sync::Sync + sync::Send>,
+        Box<dyn MultiItemModifier + sync::DynSync + sync::DynSend>,
     ),
 
     /// An AST-based derive macro.
     LegacyDerive(
         /// An expander with signature AST -> AST.
         /// The produced AST fragment is appended to the input AST fragment.
-        Box<dyn MultiItemModifier + sync::Sync + sync::Send>,
+        Box<dyn MultiItemModifier + sync::DynSync + sync::DynSend>,
     ),
 }
 

compiler/rustc_interface/src/interface.rs

@@ -60,6 +60,11 @@ impl Compiler {
     }
 }
 
+#[allow(rustc::bad_opt_access)]
+pub fn set_parallel_mode(sopts: &config::UnstableOptions) {
+    rustc_data_structures::sync::set(sopts.threads > 1);
+}
+
 /// Converts strings provided as `--cfg [cfgspec]` into a `crate_cfg`.
 pub fn parse_cfgspecs(cfgspecs: Vec<String>) -> FxHashSet<(String, Option<String>)> {
     rustc_span::create_default_session_if_not_set_then(move |_| {

compiler/rustc_lint/src/context.rs

@@ -49,9 +49,9 @@ use std::cell::Cell;
 use std::iter;
 use std::slice;
 
-type EarlyLintPassFactory = dyn Fn() -> EarlyLintPassObject + sync::Send + sync::Sync;
+type EarlyLintPassFactory = dyn Fn() -> EarlyLintPassObject + sync::DynSend + sync::DynSync;
 type LateLintPassFactory =
-    dyn for<'tcx> Fn(TyCtxt<'tcx>) -> LateLintPassObject<'tcx> + sync::Send + sync::Sync;
+    dyn for<'tcx> Fn(TyCtxt<'tcx>) -> LateLintPassObject<'tcx> + sync::DynSend + sync::DynSync;
 
 /// Information about the registered lints.
 ///
@@ -169,7 +169,7 @@ impl LintStore {
 
     pub fn register_early_pass(
         &mut self,
-        pass: impl Fn() -> EarlyLintPassObject + 'static + sync::Send + sync::Sync,
+        pass: impl Fn() -> EarlyLintPassObject + 'static + sync::DynSend + sync::DynSync,
     ) {
         self.early_passes.push(Box::new(pass));
     }
@@ -182,7 +182,7 @@ impl LintStore {
     /// * See [rust-clippy#5518](https://github.com/rust-lang/rust-clippy/pull/5518)
     pub fn register_pre_expansion_pass(
         &mut self,
-        pass: impl Fn() -> EarlyLintPassObject + 'static + sync::Send + sync::Sync,
+        pass: impl Fn() -> EarlyLintPassObject + 'static + sync::DynSend + sync::DynSync,
     ) {
         self.pre_expansion_passes.push(Box::new(pass));
     }
@@ -191,8 +191,8 @@ impl LintStore {
         &mut self,
         pass: impl for<'tcx> Fn(TyCtxt<'tcx>) -> LateLintPassObject<'tcx>
         + 'static
-        + sync::Send
+        + sync::DynSend
-        + sync::Sync,
+        + sync::DynSync,
     ) {
         self.late_passes.push(Box::new(pass));
     }
@@ -201,8 +201,8 @@ impl LintStore {
         &mut self,
         pass: impl for<'tcx> Fn(TyCtxt<'tcx>) -> LateLintPassObject<'tcx>
         + 'static
-        + sync::Send
+        + sync::DynSend
-        + sync::Sync,
+        + sync::DynSync,
     ) {
         self.late_module_passes.push(Box::new(pass));
     }

compiler/rustc_lint/src/late.rs

@@ -16,7 +16,7 @@
 
 use crate::{passes::LateLintPassObject, LateContext, LateLintPass, LintStore};
 use rustc_ast as ast;
-use rustc_data_structures::sync::join;
+use rustc_data_structures::sync::{join, DynSend};
 use rustc_hir as hir;
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::intravisit as hir_visit;
@@ -429,7 +429,7 @@ fn late_lint_crate_inner<'tcx, T: LateLintPass<'tcx>>(
 /// Performs lint checking on a crate.
 pub fn check_crate<'tcx, T: LateLintPass<'tcx> + 'tcx>(
     tcx: TyCtxt<'tcx>,
-    builtin_lints: impl FnOnce() -> T + Send,
+    builtin_lints: impl FnOnce() -> T + Send + DynSend,
 ) {
     join(
         || {

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -8,7 +8,7 @@ use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::{FxHashMap, FxIndexSet};
 use rustc_data_structures::memmap::{Mmap, MmapMut};
 use rustc_data_structures::stable_hasher::{Hash128, HashStable, StableHasher};
-use rustc_data_structures::sync::{join, par_iter, Lrc, ParallelIterator};
+use rustc_data_structures::sync::{join, par_for_each_in, Lrc};
 use rustc_data_structures::temp_dir::MaybeTempDir;
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
@@ -2125,7 +2125,7 @@ fn prefetch_mir(tcx: TyCtxt<'_>) {
         return;
     }
 
-    par_iter(tcx.mir_keys(())).for_each(|&def_id| {
+    par_for_each_in(tcx.mir_keys(()), |&def_id| {
         let (encode_const, encode_opt) = should_encode_mir(tcx, def_id);
 
         if encode_const {

compiler/rustc_middle/src/hir/map/mod.rs

@@ -5,7 +5,7 @@ use rustc_ast as ast;
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::svh::Svh;
-use rustc_data_structures::sync::{par_for_each_in, Send, Sync};
+use rustc_data_structures::sync::{par_for_each_in, DynSend, DynSync};
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId, CRATE_DEF_ID, LOCAL_CRATE};
 use rustc_hir::definitions::{DefKey, DefPath, DefPathData, DefPathHash};
@@ -150,11 +150,6 @@ impl<'hir> Map<'hir> {
         self.tcx.hir_module_items(module).items()
     }
 
-    #[inline]
-    pub fn par_for_each_item(self, f: impl Fn(ItemId) + Sync + Send) {
-        par_for_each_in(&self.tcx.hir_crate_items(()).items[..], |id| f(*id));
-    }
-
     pub fn def_key(self, def_id: LocalDefId) -> DefKey {
         // Accessing the DefKey is ok, since it is part of DefPathHash.
         self.tcx.definitions_untracked().def_key(def_id)
@@ -502,7 +497,7 @@ impl<'hir> Map<'hir> {
     }
 
     #[inline]
-    pub fn par_body_owners(self, f: impl Fn(LocalDefId) + Sync + Send) {
+    pub fn par_body_owners(self, f: impl Fn(LocalDefId) + DynSend + DynSync) {
         par_for_each_in(&self.tcx.hir_crate_items(()).body_owners[..], |&def_id| f(def_id));
     }
 
@@ -640,7 +635,7 @@ impl<'hir> Map<'hir> {
     }
 
     #[inline]
-    pub fn par_for_each_module(self, f: impl Fn(LocalDefId) + Sync + Send) {
+    pub fn par_for_each_module(self, f: impl Fn(LocalDefId) + DynSend + DynSync) {
         let crate_items = self.tcx.hir_crate_items(());
         par_for_each_in(&crate_items.submodules[..], |module| f(module.def_id))
     }

compiler/rustc_middle/src/hir/mod.rs

@@ -9,7 +9,7 @@ pub mod place;
 use crate::ty::query::Providers;
 use crate::ty::{EarlyBinder, ImplSubject, TyCtxt};
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
-use rustc_data_structures::sync::{par_for_each_in, Send, Sync};
+use rustc_data_structures::sync::{par_for_each_in, DynSend, DynSync};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::*;
 use rustc_query_system::ich::StableHashingContext;
@@ -77,19 +77,19 @@ impl ModuleItems {
         self.owners().map(|id| id.def_id)
     }
 
-    pub fn par_items(&self, f: impl Fn(ItemId) + Send + Sync) {
+    pub fn par_items(&self, f: impl Fn(ItemId) + DynSend + DynSync) {
         par_for_each_in(&self.items[..], |&id| f(id))
     }
 
-    pub fn par_trait_items(&self, f: impl Fn(TraitItemId) + Send + Sync) {
+    pub fn par_trait_items(&self, f: impl Fn(TraitItemId) + DynSend + DynSync) {
         par_for_each_in(&self.trait_items[..], |&id| f(id))
     }
 
-    pub fn par_impl_items(&self, f: impl Fn(ImplItemId) + Send + Sync) {
+    pub fn par_impl_items(&self, f: impl Fn(ImplItemId) + DynSend + DynSync) {
         par_for_each_in(&self.impl_items[..], |&id| f(id))
     }
 
-    pub fn par_foreign_items(&self, f: impl Fn(ForeignItemId) + Send + Sync) {
+    pub fn par_foreign_items(&self, f: impl Fn(ForeignItemId) + DynSend + DynSync) {
         par_for_each_in(&self.foreign_items[..], |&id| f(id))
     }
 }

compiler/rustc_middle/src/ty/context.rs

@@ -496,7 +496,7 @@ pub struct GlobalCtxt<'tcx> {
     ///
     /// FIXME(Centril): consider `dyn LintStoreMarker` once
     /// we can upcast to `Any` for some additional type safety.
-    pub lint_store: Lrc<dyn Any + sync::Sync + sync::Send>,
+    pub lint_store: Lrc<dyn Any + sync::DynSync + sync::DynSend>,
 
     pub dep_graph: DepGraph,
 
@@ -648,7 +648,7 @@ impl<'tcx> TyCtxt<'tcx> {
     /// reference to the context, to allow formatting values that need it.
     pub fn create_global_ctxt(
         s: &'tcx Session,
-        lint_store: Lrc<dyn Any + sync::Send + sync::Sync>,
+        lint_store: Lrc<dyn Any + sync::DynSend + sync::DynSync>,
         arena: &'tcx WorkerLocal<Arena<'tcx>>,
         hir_arena: &'tcx WorkerLocal<hir::Arena<'tcx>>,
         untracked: Untracked,

compiler/rustc_middle/src/ty/context/tls.rs

@@ -94,8 +94,8 @@ where
         f(None)
     } else {
         // We could get an `ImplicitCtxt` pointer from another thread.
-        // Ensure that `ImplicitCtxt` is `Sync`.
+        // Ensure that `ImplicitCtxt` is `DynSync`.
-        sync::assert_sync::<ImplicitCtxt<'_, '_>>();
+        sync::assert_dyn_sync::<ImplicitCtxt<'_, '_>>();
 
         unsafe { f(Some(downcast(context))) }
     }

compiler/rustc_middle/src/ty/list.rs

@@ -199,6 +199,12 @@ impl<'a, T: Copy> IntoIterator for &'a List<T> {
 
 unsafe impl<T: Sync> Sync for List<T> {}
 
+// We need this since `List` uses extern type `OpaqueListContents`
+#[cfg(parallel_compiler)]
+use rustc_data_structures::sync::DynSync;
+#[cfg(parallel_compiler)]
+unsafe impl<T: DynSync> DynSync for List<T> {}
+
 // Safety:
 // Layouts of `Equivalent<T>` and `List<T>` are the same, modulo opaque tail,
 // thus aligns of `Equivalent<T>` and `List<T>` must be the same.

compiler/rustc_middle/src/ty/query.rs

@@ -52,9 +52,8 @@ use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap, FxIndexSet};
 use rustc_data_structures::steal::Steal;
 use rustc_data_structures::svh::Svh;
-use rustc_data_structures::sync::AtomicU64;
+use rustc_data_structures::sync::AtomicU64;use rustc_data_structures::sync::WorkerLocal;
-use rustc_data_structures::sync::Lrc;
+use rustc_data_structures::sync::{self, Lrc};
-use rustc_data_structures::sync::WorkerLocal;
 use rustc_data_structures::unord::UnordSet;
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir as hir;

compiler/rustc_session/src/cstore.rs

@@ -207,7 +207,7 @@ pub trait MetadataLoader: std::fmt::Debug {
     fn get_dylib_metadata(&self, target: &Target, filename: &Path) -> Result<MetadataRef, String>;
 }
 
-pub type MetadataLoaderDyn = dyn MetadataLoader + Send + Sync;
+pub type MetadataLoaderDyn = dyn MetadataLoader + Send + Sync + sync::DynSend + sync::DynSync;
 
 /// A store of Rust crates, through which their metadata can be accessed.
 ///
@@ -252,7 +252,7 @@ pub trait CrateStore: std::fmt::Debug {
     fn import_source_files(&self, sess: &Session, cnum: CrateNum);
 }
 
-pub type CrateStoreDyn = dyn CrateStore + sync::Sync + sync::Send;
+pub type CrateStoreDyn = dyn CrateStore + sync::DynSync + sync::DynSend;
 
 pub struct Untracked {
     pub cstore: RwLock<Box<CrateStoreDyn>>,

compiler/rustc_span/src/source_map.rs

@@ -14,7 +14,7 @@ pub use crate::*;
 
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::stable_hasher::{Hash128, Hash64, StableHasher};
-use rustc_data_structures::sync::{AtomicU32, Lrc, MappedReadGuard, ReadGuard, RwLock};
+use rustc_data_structures::sync::{AtomicU32, IntoDyn, Lrc, MappedReadGuard, ReadGuard, RwLock};
 use std::cmp;
 use std::hash::Hash;
 use std::path::{self, Path, PathBuf};
@@ -176,7 +176,7 @@ pub struct SourceMap {
     used_address_space: AtomicU32,
 
     files: RwLock<SourceMapFiles>,
-    file_loader: Box<dyn FileLoader + Sync + Send>,
+    file_loader: IntoDyn<Box<dyn FileLoader + Sync + Send>>,
     // This is used to apply the file path remapping as specified via
     // `--remap-path-prefix` to all `SourceFile`s allocated within this `SourceMap`.
     path_mapping: FilePathMapping,
@@ -202,7 +202,7 @@ impl SourceMap {
         SourceMap {
             used_address_space: AtomicU32::new(0),
             files: Default::default(),
-            file_loader,
+            file_loader: IntoDyn(file_loader),
             path_mapping,
             hash_kind,
         }

src/librustdoc/lib.rs

@@ -730,6 +730,9 @@ fn main_args(at_args: &[String]) -> MainResult {
         }
     };
 
+    // Set parallel mode early as the error handler will already create locks.
+    interface::set_parallel_mode(&options.unstable_opts);
+
     let diag = core::new_handler(
         options.error_format,
         None,